REPORT OF THE DIRECTOR I9I7 163 



surface and in the interstices of which occurred barite crystals of 

 several generations and types. These have been judged of sufficient 

 interest to warrant a short description. 



Type 1. Crystals of this type are the largest as well as the simplest 

 of those under consideration. They average 10 mm in length, measured 

 on the a axis, and are tabular prisms bounded by the planes of 

 c (001) and m (no) in relative habit-development of about 8:1. 

 The crystals are sharply defined, transparent and show parallel 

 bands of inclusions which, although following in the main the outlines 

 of the prism m, also parallel the macrodome d (102) and the prism 

 X (210), forms absent from this type although occurring in well- 

 developed habit on the crystals of type 2. This evidence would 

 suggest that the type under present consideration represents a later 

 generation of barite formed around preexisting crystals, possibly of 

 type 2. Figure 5 illustrates this type. 



Type 2. A large proportion of the barite crystals occurring 

 implanted on the botryoidal masses of psilomelane are representative 

 of this type. They are of prismatic habit, thicker parallel to the 

 c axis than those of type 1, and have for the dominant prisms X (210) 

 and m (no), the former in rather larger development, giving to the 

 crystals a thin, pointed aspect. The planes are roughened by etch 

 pits, but give sufficiently good reflections to admit of their being 

 easily identified, and in general the type seems to represent an early 

 stage of barite deposition. The observed forms are: 



c (001), a (100), b (010), X (210), m (no), X (130), 



(on), d (102), z (in). 



Figure 6 represents the habit of this type. 



Type 3. Crystals representative of this type are of a later gene- 

 ration than those of the two preceding, and are probably the result 

 of solution and recrystallization from individuals of a previous 

 genetic phase. In habit they resemble the crystals of type 2, but 

 are more transparent than these latter and are also characterized by 

 smoother and more highly reflective faces, characteristics which 

 argue in favor of a slow process of crystallization under more or less 

 ideal conditions. The observed forms are: 



a (100), b (010), c (001), 2 3 (5 IO )*> ^ (210), y) (320), 



m (no), N (230), F (380)*, B (370), X (130), d (102), 



A (013), o (on), z (in),/ (113), q (114), ("S). X * ( 2I 5)-* 



The forms marked with an asterisk (*) are new. 



S3 (510). This new prism was observed twice on crystal II and 

 once each on crystals V and VI of the series measured. The reflec- 

 6 



